So now that I hatched my duracrete slug egg, is there a way for it to earn xp in combat even though it doesn't have weapons? Or do I need to have it patrol or something to level it up? I only ask as it does in fact have a level bar which would imply that it could be leveled.

There are still some plans to make further modifications to creatures that Jesfa was wanting to work on, so perhaps some of the currently defenseless creatures will be able to gain XP from combat in the future as well.